The reproduction of the first two octaves of audible low-frequency sound, from 20 to 80 Hz, is a long-standing problem in that large volumes of air must be moved. For a given loudness of a reproduced low-frequency sound, the air volume moved must be doubled for each halving of reproduced sound frequency.
Many solutions have been attempted. Low-frequency voice-coil-and-cone loudspeakers with highly compliant suspensions have been used. However, the long cone travels needed are difficult to achieve with linearity. Nonlinearities introduce intermodulation distortion, the modulation of higher reproduced frequencies by lower reproduced frequencies. More serious attempts to produce the lowest frequencies in sound isolate the lower two octaves from higher frequencies for reproduction through "subwoofer" loudspeakers.
Direct radiator voice-coil-and-cone loudspeakers, which have at best an efficiency of a few percent, have been used as subwoofers in 12 to 30 inch frame sizes in large cabinets of several cubic feet in volume. Required driving powers are also large, typically several hundred watts. Subwoofer low-frequency response is limited by resonance of the voice-coil-and-cone masses in combination with compliance of the voice-coil support "spider", the cone flexible surround, and the air compliance of a closed cabinet, if used. Together these set an effective lower limit to the frequencies of output sound since it is difficult to drive a loudspeaker below its low-frequency resonance. Closed-loop (negative feedback) servomechanisms controlling voice-coil movements have been used to reduce these effects but they may leave the cone and flexible surround free to flex in unwanted modes at high amplitude, limiting usable power output by the onset of distortion.
In auditoria and stadia, horn-type radiators driven by fairly conventional voice-coil-and-cone transducers have also been used as subwoofers with efficiencies in the 0 to 50 percent range. Their limitation in the domestic environment is that for reasonable performance the perimeter of the horn mouth must be of the order of the wavelength of the lowest reproduced sound, e.g., over 50 feet at 20 Hz. Recently, horns of the type disclosed in U.S. Pat. No. 4,564,727 have been driven from externally cooled servomotors through pulley, belt, and cone arrangements, achieving remarkable sustained acoustic outputs.
Most recently, in U.S. Pat. No 4,763,358, the use of a positive-displacement rotary-vane pump is disclosed. If of suitable size it should be able to produce usable output to and below the 20 Hz limit of audibility. The rotary vane pump may be used to drive a horn, though the required horn mouth size (above) practically excludes it from the domestic user environment. If used as a direct radiator, rotary vane pump efficiency is similar to that of voice-coil-and-cone direct radiators. At high output, most of the input power must be dissipated as heat, usually from within a closed cabinet. Though their volumetric efficiency is high relative to that of voice-coil-and-cone loudspeakers, since much of rotary vane device volume may be swept by the vanes, the devices and their cabinetry can be large. Cabinetry structure for devices having only full-length ports in pump-enclosure sidewalls is awkward. Motor wear and noise, bearing noise, and seal-leakage noise can be problematic in a quiet, e.g., home or auto, user environment. Port-turbulence noise must be managed--a nominal 15 inch diameter, 9 inch long rotary acoustic radiator moves about 6 times the air volume in a single stroke as does a conventional 15 inch loudspeaker.
Position sensing has heretofore been disclosed for rotary acoustic radiators to provide negative feedback information active in the same frequency band as the acoustic output and linearize vane travel. This approach has been used successfully in voice-coil-and-cone loudspeakers, which are linear at null or neutral position. Rotary acoustic transducers are not linear at null. The support bearings of rotary devices have static friction differing severalfold from dynamic friction, and both static and dynamic friction vary with temperature and time. Dynamic friction, determined more by grease seals than by the bearings themselves, increases with rotational velocity.
Bearing, slip ring, and motor-brush static friction induce distortion at low output amplitudes. Total breakaway (from stop) torques, which are typically 2 percent of full motor torque, are 20 percent of torque when the audio output level is down 20 db, and further increase the relative distortion level with decreasing output. Since the usual dynamic range of entertainment audio is 40 to 50 db, such distortions at mid and low amplitudes are serious problems. Commutation discontinuities and irregularities of motor magnetic fields also contribute somewhat unpredictably to low-amplitude output distortion, as their magnitude is often a discontinuous function of motor armature rotational position.
Negative feedback adequate to contain these nonlinearities to a user-acceptable level would likely be 14 db or more, implying a corollary unity-gain negative feedback loop crossover in the region of 300 Hz or higher. Stability is difficult to assure over a subwoofer's life with such nonlinear electromechanical components and high bandwidth in a negative feedback loop.
Commutated motors, when used to drive acoustic transducers, introduce a special problem. A wide variety of techniques have been employed to reduce the characteristic of commutated motors commonly referred to as torque cogging or torque ripple, hereinafter referred to as torque ripple, which is the principal distortion-generating limitation of commutated motors when employed in rotary acoustic transducers. These ripple effects occur when windings connected to rotationally adjacent commutator sectors are shunted together by brushes. Brushless motors having multipole permanent magnet rotors and multiphase stator windings, particularly those having precision angular position information available for use in commutation such as computer memory disk drives and the rotary acoustic transducer of this invention, can be commutated without the positional uncertainty and torque ripple arising through use of mechanical brushes. With electronic commutation angular gaps may be introduced between stator connections during commutation to minimize inductive and ferromagnetic hysteresis effects in stator windings during phase connection and disconnection, as in Janssen U.S. Pat. No. 4,703,236. Separate windings for each pole set in a multiphase motor which share a single driving source, such as a power amplifier, can limit the electromagnetic disturbance during an event of commutation to one or two pole sets, rather than disturbing the entire stator during each commutation as in conventional lap or wave stator finding patterns wherein all stator poles share the same winding circuit.
Torque ripple is reduced by the accurate commutation described hereinabove. Nevertheless in brushless motors stator winding commutation generally occurs adjacent to rotor pole edges and is a source of torque ripple as stator pole magnetic flux reverses and stator pole flux transfers from one rotor pole to the next. Additional sources of torque ripple are cogging of the rotor from pole to pole of the stator due to uneven flux distribution cross pole faces and winding slots, flux variations across the faces of stator poles themselves, and pole saturation. These sources of torque ripple have been compensated by using large numbers of stator poles; using numbers of rotor and stator poles which are not multiples or submultiples of each other; skewing the stator poles from their usual radial or axial alignments in axial-gap and radial-gap motors, respectively; shaping the stator pole faces, as with surface depressions, to produce a desired flux distribution, as in Hertrich, U.S. Pat. No. 4,874,975; and modulating the stator winding drive current with a repetitive pattern in synchrony with the multipole rotor assembly angular rotation over stator poles, as in Nakase et al., U.S. Pat. No. 4,525,657.
There is therefore a need to address these problems of low-frequency sound transducers, and in particular rotary-vane transducers, to produce a low-frequency sound reproducing apparatus and method more suitable for the consumer environment.